1. Field of the Invention
This invention relates to static eliminators or neutralizers and more particularly relates to corona discharge devices in which an A.C. power supply has its high voltage side connected to the discharge electrodes, usually of pointed disposition, and the other side connected to a conductive member or casing adjacent to the discharge electrodes so that both positive and negative ions are emitted. These dual polarity ions are utilized to neutralize the surfaces of articles electrostatically charged by frictional, mechanical, electrical or other generated forces. The present invention is especially concerned with static eliminator systems in which ion emission can be adjustably controlled to produce an equal number of positive and negative ions or a predominance of ions of a particular polarity.
2. Prior Art
Static eliminators are devices for producing both positive and negative ions in order to neutralize articles or materials which are charged to a particular polarity or which have some of both the positive and negative charges on various portions of their surfaces with a net residual charge in certain zones. When an A.C. high voltage of fairly high magnitude, for example 15,000 volts, is applied across the discharge points and the grounded casing or shield of such static eliminators, positive and negative ions are emitted from the discharge electrodes. While positive and negative ion production may be precisely equal under certain circumstances, in most instances one or the other polarity of ions will predominate depending upon (1) the manner in which the high voltage is connected to the ionizing points, i.e. whether the points are resistively coupled as in a directly connected bar or capacitively coupled as in a "shockless" bar, (2) the geometry of the static bar, in particular the configuration of the grounded portions of the bar and the relationship thereof with respect to the ionizing points and (3) the distance between the static bar and the material to be discharged, as well as the presence of adjacent grounds with respect to the bar, the latter affecting the amounts of the respective positive or negative ions being emitted actually reaching the charged material.
In the direct connected static bar, there is usually a predominance of negative ions produced, even though the discharge points are connected to an A.C. source whose positive and negative voltage is of equal amplitude. The excess negative ion production is as a result of the greater mobility of such negative ions and also because of the inherent characteristics of corona formation wherein ionization occurs over a greater portion of the negative half cycle of voltage in relation to the ionization occurring during the comparable positive half cycle.
In the case of the capacitively coupled static bar, there is usually a predominance of positive ions emitted, the greater positive ion production resulting from the fact that a D.C. voltage is developed across the capacitance in the direction which biases the points slightly positively. That is, in a capacitively coupled system, the characteristic of a point to produce more negative ions during the negative half cycle of imposed voltage causes the capacitance to charge to a positive D.C. voltage which adds algebraically to the A.C. voltage. Hence, the voltage on a point with respect to the casing is greater during the positive half cycle than during the negative half cycle, thereby causing excess positive ions to be emitted in a capacitively coupled bar.
Accordingly, if the material to be discharged lies upon or is adjacent to a grounded (or other) surface, the material may charge up to the polarity of the predominantly positive charge produced by the capacitively coupled bar or to the predominantly negative charges emitted by the direct coupled bar. Where a neutral article or sheet is required, of course it is desirable to have an equal number of positive and negative ions. However, under other circumstances, it is frequently important to charge a sheet just a slight amount either positively or negatively, as when a subsequent sheet sliding upon a pile would frictionally charge the sheet below even though both were neutral prior to stacking. Here, one would wish to cause emission in which ions of one or the other polarity would slighty predominate such that a sheet flowing onto a stack would be previously charged oppositely and to the extent necessary just to neutralize the charge which that sheet would produce in sliding frictionally upon the sheet below.
In U.S. Pat. No. 4,093,543, there is shown a balanced emission system for capacitively coupled (shockless) static eliminators wherein pointed conductive needles connected to the ground side of the A.C. high voltage source are adjacently spaced from and are in interacting disposition with at least some of the pointed discharge electrodes. The points of the "balancing" or control needles are adapted to be adjustably positioned with respect to the discharge electrodes so that an equal number of ions of each polarity are emitted. This provides in effect a mechanical balancing system.
Another method for balancing the number of positive and negative ions emitted was to provide two static bars or two electrodes in a single housing and supplying one bar or electrode with a D.C. voltage of one polarity and the other with a D.C. voltage of the opposite polarity. By controlling the voltage on the respective electrodes, the precise mixture of ions could be generated. However, a D.C. system precludes the use of a capacitive coupling and therefore obviates a shockless static bar construction
In U.S. Pat. No. 2,879,395, equalization of ion production for each polarity was accomplished by incorporation a small D.C. power supply either between the casing and ground or between the A.C. generator and ground. This miniature D.C. power supply functioned by placing a D.C. bias of the appropriate polarity on the casing or on the discharge points and was connected in such a way as to retard the output of ions of the opposite polarity. Adjustment of the magnitude of the auxiliary D.C. voltage provided the desired balance of positive and negative ion emission. However, the use of an auxiliary D.C. generator has the disadvantage of requiring a separate power supply, thus making the arrangement expensive and bulky. Furthermore, use of a D.C. power supply in a capacitively coupled static bar requires that it be incorporated in the casing circuit thus causing the latter to be "hot" and necessitating insulation.
In U.S. Pat. No. 3,714,531, the device for controlling the ratio of positive and negative ions is for a direct coupled system only. The controlling circuitry is incorporated in the secondary of the high voltage transformer in one of two ways, (1) by decreasing the negative component of the voltage through the use of a diode in series with the high voltage coil or (2) by increasing the positive component of the A.C. voltage applied to the points through the use of a pair of secondary coils, one being connected directly to the points while the other secondary coil includes a diode and a capacitor in series therewith so as to add to the positive voltage of the first coil. The D.C. on the core of the variable secondary coil changes the permeability of the core and distorts voltage on the other secondary.
It is therefore an object of this invention to provide a static eliminator system in which ion emission can be regulated to produce equal quantities of positive and negative ions or in the alternative a predominance of ions of one polarity.
Another object of this invention is to provide a controlled emission static eliminator in which ion emission can easily be regulated regardless of whether the discharging points are directly or capacitively coupled to the A.C. high voltage source.
Yet another object of this invention is to provide an R-C electrical biasing circuit for the primary of a high voltage transformer in order to minimize size and insulation requirements of the components in a controlled emission static eliminator system.
Other objects of this invention are to provide an improved device of the character described that is easily and economically produced, sturdy in construction and both highly efficient and effective in operation.